U.S. Pat. No. 3,031,304 discloses a method of preparing fine grain nuclear track emulsions utilizing two Zenith gear pumps which pump mineral oil to displace the respective silver and salt solutions from separatory funnels into an aqueous gelatin solution contained in a reaction vessel. The silver ion concentration of the solution is followed potentiometrically with a silver electrode and a calomel electrode connected with salt bridges. Limit switches are wired into the potentiometric circuit so that when the pAg rises above a preselected level, a third pump subtracts 1% of the halide ion solution being forced by displacement from the separatory funnel containing the halide solution. The pAg varies widely at first, but after a few minutes the silver ion concentration becomes relatively steady. A fourth halide solution pump may be provided for manual operation in order that the halide ion supply can be properly adjusted by an operator from the observation of a potentiometer reading or from a strip recorder. It has been determined that a specific pAg can be obtained and maintained by having a specific amount of silver salt relative to a specific amount of halide salt flowing into the peptizer solution.
A prior art system for automated electronic control of silver halide emulsion preparation is that disclosed by Claes and Peelaers in Photographische Korrespondenz 102 Band, Nr 10/1967, pp. 162. In this prior art system, silver nitrate and alkali halide salt solutions are gravity fed to a precipitation vessel through two valves which are activated electronically to regulate the flow of the silver nitrate and alkali halide solutions. To keep a constant level of silver nitrate solution in what appears to be a separatory funnel, a relay is used. A recorder is used to monitor pH, pAg and also to control flow rates.
The advantages of continuously monitoring and controlling the pAg and/or the pH of the peptizer, silver salt solutions and halide salt solutions are recognized by those skilled in the art. U.S. patent application Ser. No. 194,451, now U.S. Pat. No. 3,782,954 and Ser. No. 194,466, now U.S. Pat. No. 3,785,777, filed Jan. 12, 1972, a continuation and divisional, respectively, of U.S. Pat. application Ser. No. 876,893 filed Nov. 14, 1969 and now abandoned which is a continuation-in-part of U.S. application Ser. No. 854,629 filed Sept. 2, 1969, now abandoned, all to Porter and Johnson; Ser. No. 11,838 filed Feb. 16, 1970 to Wilgus and Ser. No. 159,914 filed July 6, 1971 to Owens, both now abandoned, all make clear that during the preparation of silver halide crystals, it is desirable that the pAg and the pH of the peptizer solution which contains silver halide mixture be continuously monitored and that the pAg and pH characteristics of the solution be continuously controlled by adjusting the proportions of the silver and/or halide salt solutions being added thereto in response to changes in the pAg. The above mentioned U.S. Pat. No. 3,031,304 and the Claes and Peelaers article show typical prior art control systems.
It is recognized that the preparation of silver halide emulsions is preferably carried out in a carefully controlled environment. For example, the accurate control of rates of addition, pAg, pH, and the duration of preparation as well as temperature and the relative uniformity of mixing of the reactants as added from two separate sources in addition to other such variables is desirable. It is also well known that variations in the initial rates of delivery of silver salt or halide salt solution into a precipitation vessel can have adverse effects upon the number of nuclei formed which is directly related to the final grain size of the emulsion produced. Consequently, the final light sensitive properties of the silver halide emulsion prepared are affected by the initial delivery rates of the silver salt and halide salt solution into the precipitation vessel. Despite the above-mentioned improved prior art devices and methods, there is still a continuing need for more accurate control of variables by sophisticated and programmable metering systems that will insure the preparation of more reproducible, extremely sensitive photographic emulsions which are characterized by uniformity in grain morphology, grain size and sensitometric properties.